The present invention relates to a thermal transfer (printing) film provided with an anti-stick coating film and having excellent transfer performance. More particularly, the present invention relates to a thermal transfer (printing) film comprising a base film having a stick-preventing layer provided (laminated) on one side thereof, the stick-preventing layer being provided at a rate of 0.01 to 1.9 g/m.sup.2, the center line average roughness of the stick-preventing layer surface being 0.03 to 0.15 .mu.m, the coefficient of parallel slipping of the layer against glass surface being not more than 1.0, and the tensile strength of the film formed with the stick-preventing layer on one side thereof being not less than 8 kgf/mm.sup.2 when the fiIm is elongated by 5% in the longitudinal direction, and a heat-melting ink layer provided on the other side of the base film.
Various types of printing and recording systems are known, and among them, the thermal transfer (printing) system used for the thermal recording devices such as thermal printers has been popularly used for its excellent operatability and maintenability. In the thermal transfer (printing) system, the heat-melting ink layer provided on one side of the base film of the thermal transfer (printing) film is contacted with the recording paper, and the thermal transfer (printing) film is selectively heated by the thermal head to which pulse signals are transmitted on the side opposite from the ink layer so as to heat this ink layer through the base film to melt the inks and transfer to the recording paper.
In such thermal transfer (printing) system, it is necessary to increase input electric power for reducing input time tc the thermal head in order to attain speed-up of recording, but this has caused the following problem. It is desirable that the heating temperature by the thermal head be adjusted to a level which is below the melting point of he base film but above the melting point of the heat-melting ink layer, but when input electric power is increased, there arises the phenomenon that the base film is fused to the thermal head, whereby the feed of the thermal transfer (printing) film is prevented. This phenomenon is called sticking, and it becomes a cause of drop of recording precision, improper travelling of the film and other troubles.
In order to avoid occurrence of such sticking, a method has been employed in which a stick-preventing layer having higher heat resistance than the base film is provided on the surface of the base film opposite from the heat-melting ink layer.
Provision of such stick-preventing layer on the base film surface can indeed prevent sticking but causes the following problems.
Since the stick preventino layer is positioned together with the base film between the thermal head and the heat-melting ink layer, the heat conductive efficiency form the thermal head to the heat-melting ink layer lowers accordingly. When such heat-conductive efficiency is low there arises the necessity of further increasing input electric power to the thermal head. However, when excessively high input electric power is applied, the heat load given to the stick preventing layer itself also increases to cause wrinkling of the film or generate of the refuse of the stick preventing layer. The generated refuse is accumulated on the thermal head (printing head), thereby adversely affecting transfer precision or causing reverse transfer in the case of color printing, for instance when multiple colors such as yellow, magenta, cyan, etc., are overlapping-printed, that is, a phenomenon that when the different colors are overlapping-printed, the printed inks are taken up by the heated thermal transfer (printing) film.
Under these circumstances, a development of a thermal transfer (printing) film provided with a stick-preventing layer and having excellent heat conductivity as well as excellent transfer performance such as high transfer precision and transfer speed, which would enable transfer with a relatively low input electric power to the printing head has been demanded.
In view of the above, as a result of the present inventors' extensive studies for providing a thermal transfer (printing) film provided with a stick-preventing layer and having excellent transfer performance, it has been found that a thermal transfer (printing) film which has excellent heat conductivity and transfer performance and can well satisfy the above-mentioned requirements, and is provided with a stick preventing layer on one side of a base film and a heat-melting ink layer on the other side thereof, the stick preventing layer being provided at a rate of 0.01 to 1.9 g/m.sup.2, the center line average roughness of the stick-preventing layer surface being 0.03 to 0.15 .mu.m, the coefficient of parallel slipping against the glass surface being not more than 1.0, and the tensile strength of a film formed with the stick-preventing layer on the one side thereof being not less than 8 kgf/mm.sup.2 when the film is elongated by 5% elongation in the longitudinal direction and present invention has been attained on the basis of this finding.